1. Field of the Invention
The invention relates to a control unit for a four-wheel-drive vehicle.
2. Description of Related Art
Conventionally, there has been known a four-wheel-drive vehicle provided with main drive wheels (front wheels), to which driving force is always transmitted from a drive source, and auxiliary drive wheels (rear wheels), to which the driving force is transmitted from the drive source depending on a traveling state. The four-wheel-drive vehicle is configured such that, when the vehicle is traveling in a two-wheel-drive mode in which the driving force is transmitted only to the main drive wheels, torque transmission at both ends of a propeller shaft is interrupted to stop rotation of the propeller shaft. In this way, a power loss is reduced. Refer to, for example, Japanese Patent Application Publication No. 2011-255846 (JP 2011-255846 A).
In the four-wheel-drive vehicle described in JP 2011-255846 A, a dog clutch, which transmits torque by the engagement of meshing teeth, and a torque coupling capable of adjusting transmission torque are arranged on a torque transmission path leading to the auxiliary drive wheels. In the four-wheel-drive vehicle, the dog clutch is disposed on the main drive wheel-side of the propeller shaft, and the torque coupling is disposed on the auxiliary drive wheel-side of the propeller shaft. When the drive mode is switched from the two-wheel-drive mode to a four-wheel-drive mode, the torque that is transmitted by a driving force transmission device is increased to increase the rotational speed of the propeller shaft, the torque that is transmitted by the driving force transmission device is then decreased, and the dog clutch is engaged in a state where the transmission torque has been decreased. Thus, it is possible to suppress shocks and vibrations that are generated when the dog clutch is engaged, while reducing the time required to switch the drive mode from the two-wheel-drive mode to the four-wheel-drive mode.
In this type of four-wheel-drive vehicle, for example, when the main drive wheels slip in the two-wheel-drive mode and therefore the speed difference between the rotational speed of the main drive wheels and the rotational speed of the auxiliary drive wheels becomes large, the drive mode is switched to the four-wheel-drive mode to enhance the driving stability. However, when the main drive wheels continue slipping during switching of the drive mode to the four-wheel-drive mode, synchronization in the dog clutch cannot be achieved even if the torque that is transmitted by the driving force transmission device is increased to increase the rotational speed of the propeller shaft. As a result, it is not possible to switch the drive mode to the four-wheel-drive mode.